Antistatic sheet

ABSTRACT

According to the invention, it includes conductive pigments constituted by a substrate (chosen from the group including micas, talcum, kaolin, bentonites, montmorillionites or glass particles) which has a basic lamellar-type structure coated with an electroconductive layer of tin oxide doped with antimony. On at least one face, the sheet has a layer containing at least the conductive pigments having a basic lamellar-type structure and at least one hydrosoluble binder. 
     Application as decorative paper for laminates.

Such a sheet can be used in various fields. It may be used to makearticles for which there is a need to dissipate the electrostaticcharges produced when they are used, or for articles whose primary orsecondary function is to dissipate the electrostatic charges whichdevelop and can be dangerous in a given environment.

Flexible abrasives are known that are made up of a substrate sheet ontowhich abrasive particles are glued with an adhesive, so that it isnecessary to dissipate the electrostatic charges that arise during theiruse.

In effect, unless the abrasive is treated to dissipate the charges, thedust formed during the abrasion of an object is deposited and crushesthe abrasive particles; this lowers the yield from the abrasion process.Moreover, workers can suffer electric shocks, which can causeuncontrollable gestures and put them in danger on the job.

Laminates are also known, which are currently used in making furniture,work tables, wall panels and the like.

The products obtained are used in particular in operating rooms, whiterooms or computer rooms. In these rooms, there is a need to prevent dustfrom being retained, particularly as a result of the attraction exertedby electrostatic charges, and there is also a need to avoid suddenelectrostatic discharge; hence the surfaces located in the room must betreated to provide for regular dissipation of the static electricity.

In the field of abrasives, the substrate or the adhesive layer, or thesurface of the abrasive particles, is treated with a conductive product.Conductive products that have been employed are quaternary ammoniumsalts, carbon black, metal powders or fibers, metal alloys, metal salts,doped conductive polymers, or mineral pigments made conductive bycoating them with an electroconductive layer of metal oxide.

It is known to use carbon black, either in bulk or on the surface, tomake a conductive substrate.

In European Patent Application EP A 414 494, the abrasive is madeconductive by incorporating carbon black in the adhesive used for gluingthe abrasive particles.

In U.S. Pat. No. 3,942,959, the abrasive is made conductive by a layerof conductive compound, which may be a metal, metal alloy, metalpigment, salt or metal complex, this layer being deposited between twoinsulating layers. The conductive product may be placed on the back ofthe substrate, or on the face of the substrate (under the adhesive), ormixed with the adhesive or the particles.

In French Patent Application FR A 2276144, an abrasive is madeconductive by depositing a conductive layer on top of the abrasiveparticles, the conductive product being graphite in particular.

In European Patent Application EP A 408 943, the abrasive is madeconductive by treating the surface of the layer containing the abrasiveparticles with a solution containing a quaternary ammonium salt.

In European Patent Application EP A 398 580, the abrasive is madeconductive by treating the surface of the layer containing the abrasiveparticles with a doped conjugated polymer, such as polythiophene,polyaniline, or polypyrrole.

In the field of laminates, the sheets of paper constituting them aretreated with a conductive product.

First, Applicants wish to reiterate how laminates are manufactured ingeneral, by distinguishing the two types of currently existinglaminates, that is, those known as high-pressure laminates and thoseknown as low-pressure laminates.

High-pressure laminates are produced from a core made up of a stack ofsheets, generally of kraft paper, impregnated with heat-hardenableresin, in particular a phenolic resin.

Once the sheets of kraft paper have been impregnated with resin, theyare dried and cut and then stacked on one another; the number of stackedsheets depends on the intended uses, and in general varies between 3 and9.

Next, a decorative sheet is put in place; it may be combined withprinted patterns or may be given an iridescent or metallic appearanceand is impregnated with a heat-hardenable resin that does not darkenwhen exposed to heat (a melamine-formaldehyde resin, for instance). Aprotective covering sheet or overlay, also impregnated with a resin butwithout a pattern and being transparent in the final laminate, issometimes placed on top of the decorative sheet.

The stack of the various types of impregnated sheets is placed in apress provided with a metal sheet that imparts its surface appearance;the assembly is laminated under pressure and heat; an extremely hardunitary structure is obtained that has a decorative effect.

Low-pressure laminates are produced in a manner similar to high-pressurelaminates, but the lamination of the decorative sheets is done directlyon a wood particle board panel or any other basic substrate.

A third type of product exists, the finish foil, which also belongs tothe category of decorative papers. This paper sheet, which ispreimpregnated or postimpregnated (generally with a mixture of latex andmelamine-formaldehyde resin) is intended to be glued onto a particleboard or any other substrate.

In French Patent Application FR-A 2540041, a laminate is made conductiveby making some of the sheets constituting the core conductive, byincorporating an electroconductive material such as carbon black, ametal, metal salts, or conductive fibers into each sheet.

In French Patent Application FR-A 2557167, conductive fibers aredispersed in a paper to obtain a conductive laminate. These fibers arecarbon fibers, metal fibers or fibers covered with a metal.

In Japanese Patent Application JP-A 58 034 861, a conductive pigment isdescribed as an antistatic agent for plastics, the conductive pigmentbeing obtained by depositing tin or indium onto a basic mineral,nonconductive pigment, followed by heating in an oxygen atmosphere toform its oxide. Hence the metal oxide is not doped.

Numerous pigments are named as being usable basic pigments, but noadvantage whatsoever associated with a particular pigment or ageometrical form of a family of pigments is mentioned.

In U.S. Pat. No. 5,071,676, a conductive pigment is described that canbe used to lend antistatic properties to cardboard papers. This pigmentis constituted by a nonconductive substrate covered with anelectroconductive tin oxide layer doped with antimony, which in turn iscovered with a layer that lends the pigment an isoelectric point between5 and 9 in order to facilitate its dispersion. The substrate may be ofany arbitrary kind and is not critical to that invention.

European Patent Application A 415478 describes a colored and conductivepigment that can be used in laminated papers.

This pigment is constituted by a basic pigment of rutile and mixed-phasetitanium dioxide, coated with a layer of tin oxide doped with antimony.The rutile has a spherical geometric form; hence this pigment isspherical.

All these conductive products mentioned above have variousdisadvantages.

The major disadvantage of carbon black is that black products areobtained, which may be a problem from an aesthetic standpoint and meansthat the products cannot be printed in the way one might wish.

A disadvantage of certain products, such as doped polymers, is theirhigh cost.

A disadvantage of certain products, such as quaternary ammonium salts,is that it gives the articles excessively low conductivity, for the sakeof a good flow of the electric static charges.

Another disadvantage of conductive salts is that the level ofconductivity in the products containing them varies with the relativehumidity.

A disadvantage of certain conductive products, such as aluminum, is thesensitivity to water; in the presence of water, a dangerous discharge ofhydrogen occurs. Hence they cannot easily be used in an aqueous medium.

Disadvantages associated with the use of conductive fibers are theveined or tinted appearance given to the paper, especially when carbonfibers are used, on the one hand, and the worsening of the physicalcharacteristics of the sheet of paper on the other.

Certain mineral conductive pigments that are made conductive by a layerof metal oxide can lend an overly slight conductivity in order to assuregood dissipation of the electrostatic charges, especially when employedin a papermaking application.

The applicant proposes to overcome the aforementioned disadvantages.

The object of the invention is to furnish a sheet of paper that has asufficient level of electrical conductivity to lend it antistaticproperties.

One skilled in the art knows by experience that if an article is toeffectively dissipate electrostatic charges, its surface resistivityshould preferably be no higher than approximately 10⁷ ohms, measured byASTM Standard 257-66.

A second object is to furnish a sheet having antistatic properties thatcan be made entirely in an aqueous medium.

A third object is to furnish a sheet having antistatic properties thatdo not vary with the relative humidity.

A fourth object is to furnish a sheet having antistatic properties whichhas a neutral aesthetic appearance; that is, the product that makes thesheet conductive should change the appearance of the sheet onlyslightly, if at all.

Another object is to furnish a sheet having antistatic properties whosemechanical characteristics are good.

A further object is to furnish a sheet having antistatic propertieswhich has a relatively low cost.

The applicant has found that the objects of the invention are attainedby making a sheet which includes conductive pigments having a basiclamellar-type structure and which are provided with an electroconductivelayer of doped metal oxide.

As mentioned above, the mineral pigments covered with doped metal oxidesare known for their electroconductive properties; however, theapplicants have discovered that, for pigments having anelectroconductive layer based on the same oxide and for the same dopant,which accordingly a priori has intrinsically comparable levels ofconductivity, the sheets of paper including these pigments have quitedifferent final conductivities depending on the basic structure(geometrical form) of the pigment substrate of the layer.

Such results are shown in accompanying Table 1; where the doped metaloxide is chosen as tin oxide doped with antimony, this doped oxide isdeposited on substrate pigments of different geometrical forms.

The conductive pigments were laid down in an aqueous medium under thesame conditions with the aid of the same binder (polyvinyl alcohol orPVA), and in the same ratio of 1:1, on the surface of a sheet of paper.

The surface resistivity of the sheets was measured by ASTM Standard257-66 for a relative humidity of 50% (the conductivity of the sheetscan be obtained by taking the inverse of the resistivity).

From this investigation, it appears that the basic structure of thepigment did have an influence on the final conductivity of the sheet.The desirable resistivity level, in order to have good dissipation ofelectrostatic charges (less than 10⁷ ohms), is attained only withpigments having a lamellar basic structure (hence a flat geometricalform); the resistivity is in fact on the order of 10⁵ ohms.

The invention accordingly furnishes a sheet of paper having antistaticproperties characterized in that it includes conductive pigments havinga basic lamellar-type structure and provided with an electroconductivelayer of doped metal oxide.

Preferably, the doped metal oxide is a tin oxide doped with antimony.

According to the invention, the pigments having a lamellar-typestructure are chosen, for example, from the group including micas,talcum, kaolin, bentonites, montmorillionites or glass particles.

In a particular embodiment of the invention, the conductive pigment is amica coated with a layer of tin oxide doped with antimony.

The mica pigments covered with a layer of antimony-doped tin oxide havegood transparency to light and do not change the aesthetic appearance ofthe paper that includes them.

In another particular embodiment of the invention, the conductivepigment is a mica covered with a layer of titanium oxide, optionally alayer of silica, and coated with a layer of tin oxide doped withantimony. These pigments have a certain iridescence, but they affect theaesthetic appearance of the paper in which they are contained onlyslightly. It may be valuable to use them in fields where the decorativeeffective of the iridescence is sought, as in the field of laminates.

The conductive pigments may be incorporated in bulk at the time thesheet is manufactured on the papermaking machine, or may be deposited onthe surface of the sheet by impregnation with a glue spreader press, orby any coating means or by printing. Preferably, the conductive pigmentsare added in an aqueous medium.

Preferably, the sheet is characterized in that on at least one face ithas a layer containing at least said conductive pigments and at leastone binder.

The binder is a binder typically used in papermaking, such ashydrosoluble binders, latexes, etc.

It may be advantageous to use a hydrosoluble binder, such as polyvinylalcohols or starches, for example, to obtain an easily repulpable sheet.

The layer may optionally include other additives typically used inpapermaking, such as viscosity-regulating agents such ascarboxymethylcellulose, anti-foaming agents, etc.

Preferably, the quantity of conductive pigments deposited onto the sheetis between 1 and 10 g/m², in dry weight.

The treated sheet is based on cellulose fibers; it may include otherorganic fibers (polyethylene, polypropylene, polyester fibers, etc.) ormineral fibers (such as glass fibers). It may also include otheradditives used in papermaking, such as fillers, gluing agents, binders,moisture resistance agents, retention agents, anti-foaming agents,viscosity-regulating agents, pH-regulating agents, and so forth.

The invention also furnishes a flexible abrasive product havingantistatic properties that is characterized in that its substrate isthis sheet with antistatic properties.

The abrasive may naturally be in sheet form but may also be in otherforms, such as a continuous strip, a disk, etc.

The sheet used preferably includes the conductive pigments on thesurface. The conductive pigments may be on the back of the abrasive oron the face that carries the particles, under the adhesive.

Since it is known to use a conductive product in the adhesive or on theparticles, the use of conductive pigments in this way can becontemplated.

The invention also relates to a decorative sheet obtained from thissheet having anti-static properties.

The invention also relates to a laminate having antistatic properties,which is characterized in that it includes at least one sheet within thelaminate is a sheet with antistatic properties. This sheet may be usedas a component of the core of the laminate, or as a decorative sheet, oroptionally as an overlay.

This sheet may also be a finish foil.

The following examples show possible embodiments of a sheet according tothe invention; they also demonstrate that the surface resistivity (henceconductivity) does not vary, or varies only very slightly, with therelative humidity.

EXAMPLE 1

On a sheet of paper, an aqueous composition of conductive pigments ofthe mica type coated with antimony-doped tin oxide, available on themarket from Merck, and starch as a binder is laid down, using a Meyerbar. The ratio of pigments to binder is 5:1.

A similar composition, but where the binder is a PVA, is laid down onanother sheet.

Samples are made having different coating weight of conductive pigments(expressed in dry weight in the table below).

The surface resistivity of each sample is measured by the ASTM Standard257-66, at relative humidity rates (abbreviated R. H.) of 50% and 20%.The results in the table below show that the resistivity of the sheetdoes not vary with the relative humidity.

The color of the sheets of papers obtained according to the inventionremains unchanged.

    __________________________________________________________________________    Binder        Starch         PVA                                              __________________________________________________________________________    Pigment coating weight (g/m.sup.2)                                                          6.0  4.6  2.2  5.7 2.75                                         Surface resistivity at 50% R.H.                                                             1.2 × 10.sup.5                                                               5.1 × 10.sup.5                                                               6.3 × 10.sup.5                                                               2 × 10.sup.5                                                                3.6 × 10.sup.5                         (ohms)                                                                        Surface resistivity at 20% R.H.                                                             1.6 × 10.sup.5                                                               3.8 × 10.sup.5                                                               6.7 × 10.sup.5                                                               2 × 10.sup.5                                                                3.7 × 10.sup.5                         (ohms)                                                                        __________________________________________________________________________

EXAMPLE 2

Samples are made as in Example 1, but a mica covered with a layer oftitanium oxide and then a layer of silica and finally coated with alayer of tin oxide doped with antimony, the ratio of tin to antimonybeing 85:15, is used as the conductive pigment. This pigment iscommercially available from Merck.

As shown by the results in the table below, it is confirmed that therelative humidity does not influence the level of conductivity of thesheets obtained by the invention.

The sheets have a slight iridescence, which has little effect on theiraesthetic appearance.

                                      TABLE 1                                     __________________________________________________________________________    Binder        Starch              PVA                                         __________________________________________________________________________    Pigment coating weight (g/m.sup.2)                                                          6.0     4.1  1.9    7.4 5.0                                     Surface resistivity at 50% R.H.                                                             0.9 × 10.sup.5                                                                  2.6 × 10.sup.5                                                               12 × 10.sup.5                                                                  6 × 10.sup.4                                                                1.3 × 10.sup.5                    (ohms)                                                                        Surface resistivity at 20% R.H.                                                             0.8 × 10.sup.5                                                                  2.6 × 10.sup.5                                                               15 × 10.sup.5                                                                  9 × 10.sup.4                                                                2.0 × 10.sup.5                    (ohms)                                                                        __________________________________________________________________________    Substrate of the                                                                       Titanium oxide                                                                          Titanium oxide     Mica/Titanium                           conductive layer                                                                       Spherical Spherical                                                                              Mica      oxide Silica                            Substrate structure                                                                    (0.02-0.1 μm)                                                                        (0.02-0.1 μm)                                                                       Lamellar  Lamellar                                __________________________________________________________________________    Quantity of                                                                            2.80 3.75 2.50                                                                              3.40 2.80 3.70 2.80                                                                              3.80                                pigments deposited,                                                           dry (g/m.sup.2)                                                               Surface resistivity                                                                    2.6 × 10.sup.9                                                               2.5 × 10.sup.8                                                               1 × 10.sup.8                                                                1.8 × 10.sup.7                                                               5.7 × 10.sup.5                                                               3.8 × 10.sup.5                                                               7 × 10.sup.5                                                                6 × 10.sup.5                  at 50% R.H. (ohms)                                                            __________________________________________________________________________

We claim:
 1. A sheet of paper having antistatic properties the surfaceresistivity (hence conductivity) of which varies slightly if at all withvariations in relative humidity, and having on at least one face of thesheet a layer containing conductive pigments and a binder, wherein theconductive pigments have a basic lamellar-type structure and areprovided with an electroconductive layer of doped metal oxide, saidsheet of paper being capable of providing antistatic properties tolaminates when used as a lamina layer thereof.
 2. The sheet of claim 1,characterized in that the conductive pigments having a lamellar-typestructure are chosen from the group consisting of micas, talcum, kaolin,bentonites, montmorillionites, and glass particles.
 3. The sheet ofclaim 1, characterized in that the doped metal oxide is a tin oxidedoped with antimony.
 4. The sheet of claim 2, characterized in that theconductive pigment is a mica coated with a layer of tin oxide doped withantimony.
 5. The sheet of claim 2, characterized in that the conductivepigment is a mica covered with a layer of titanium oxide, optionally alayer of silica, and coated with a layer of tin oxide doped withantimony.
 6. The sheet of claim 1, characterized in that the quantity ofsaid conductive pigments deposited onto the sheet is between 1 and 10g/m², in dry weight.
 7. A laminate having antistatic propertiescomprising the sheet of paper of claim
 1. 8. A laminate havingantistatic properties comprising the sheet of paper of claim
 2. 9. Alaminate having antistatic properties comprising the sheet of paper ofclaim
 3. 10. A laminate having antistatic properties comprising thesheet of paper of claim
 4. 11. A laminate having antistatic propertiescomprising the sheet of paper of claim
 5. 12. A laminate havingantistatic properties comprising the sheet of paper of claim
 6. 13. Adecorative sheet having antistatic properties, comprising the sheet ofpaper of claim
 1. 14. A decorative sheet having antistatic properties,comprising the sheet of paper of claim
 2. 15. A decorative sheet havingantistatic properties, comprising the sheet of paper of claim
 3. 16. Adecorative sheet having antistatic properties, comprising the sheet ofpaper of claim
 4. 17. A decorative sheet having antistatic properties,comprising the sheet of paper of claim
 5. 18. A decorative sheet havingantistatic properties,comprising the sheet of paper of claim
 6. 19. Aflexible abrasive comprising as its substrate a sheet of paper havingantistatic properties the surface resistivity (hence conductivity) ofwhich varies slightly if at all with variations in relative humidity,the sheet of paper having on at least one face a layer containingconductive pigments and a binder, wherein the conductive pigments have abasic lamellar-type structure and are provided with an electroconductivelayer of doped metal oxide, said sheet of paper providing antistaticproperties to the flexible abrasive.
 20. The flexible abrasive of claim19, wherein the conductive pigments having a lamellar-type structure arechosen from the group consisting of micas, talcum, kaolin, bentonites,montmorillionites and glass particles.
 21. The flexible abrasive ofclaim 19, characterized in that the doped metal oxide is a tin oxidedoped with antimony.
 22. The flexible abrasive of claim 20,characterized in that the conductive pigment is a mica coated with alayer of tin oxide doped with antimony.
 23. The flexible abrasive ofclaim 20, characterized in that the conductive pigment is a mica coveredwith a layer of titanium oxide, optionally a layer of silica, and coatedwith a layer of tin oxide doped with antimony.
 24. The flexible abrasiveof claim 19, characterized in that the quantity of said conductivepigment deposited onto the sheet is between 1 and 10 g/m², in dryweight.